1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same, which are suitable for a broadcasting television camera, a video camera, a digital still camera, a surveillance camera and a silver-halide film camera, for example.
2. Description of the Related Art
In recent years, a zoom lens having a wider angle of field, a high zoom ratio, and high optical performance is required for an image pickup apparatus such as a television camera, a silver halide film camera, a digital camera, or a video camera. As a zoom lens having a wider angle of field and a high zoom ratio, there is known a positive lead type five-unit zoom lens comprising five lens units including a positive refractive power lens unit disposed on the most object side. As this positive lead type zoom lens, there is known a five-unit zoom lens, in which a magnification-varying lens unit having functions as a variator and a compensator is constituted of three movable lens units, which move in different loci.
For instance, there is known a five-unit zoom lens suitable for a television camera, in which three movable lens units including a second lens unit having a negative refractive power, a third lens unit having a positive refractive power and a fourth lens unit having a positive refractive power perform magnification-varying and correction of image plane variation due to the magnification-varying (Japanese Patent Application Laid-Open No. 2009-128491). Other than that, there is known a five-unit zoom lens suitable for a television camera, in which three movable lens units including a second lens unit having a negative refractive power, a third lens unit having a negative refractive power and a fourth lens unit having a positive refractive power perform magnification-varying and correction of image plane variation due to the magnification-varying (Japanese Patent Application Laid-Open No. H01-126614 and Japanese Patent Application Laid-Open No. H08-234105).
Japanese Patent Application Laid-Open No. 2009-128491 discloses a zoom lens having a zoom ratio of approximately 54 and an angle of field of approximately 60 degrees at a wide-angle end. Japanese Patent Application Laid-Open No. H01-126614 discloses a zoom lens having a zoom ratio of approximately 5 and an angle of field of approximately 66 degrees at a wide-angle end. Japanese Patent Application Laid-Open No. H08-234105 discloses a zoom lens having a zoom ratio of approximately 2.5 and an angle of field of approximately 15 degrees at a wide-angle end.
In order to achieve high optical performance while maintaining a wider angle of field and a higher zoom ratio in the five-unit zoom lens, it is important to appropriately set moving conditions of the second, third, and fourth lens units as magnification-varying lens units during zooming and a refractive power of the first lens unit. Other than that, it is important to appropriately set a combined refractive power of the second and third lens units and a moving locus from the wide-angle end to an intermediate zoom position of the third lens unit. If these configurations are not appropriately set, it becomes difficult to achieve a zoom lens having a wider angle of field, a high zoom ratio, and high optical performance over the whole zoom range.
In the zoom lenses disclosed in Japanese Patent Application Laid-Open No. 2009-128491, Japanese Patent Application Laid-Open No. H01-126614 and Japanese Patent Application Laid-Open No. H08-234105, there is a tendency that the front lens effective diameter increases along with an increase of the angle of field and a tendency that variations of various aberrations due to magnification-varying increase. In addition, there is a tendency that a movement amount of a moving lens unit for magnification-varying increases along with an increase of the zoom ratio, and hence the total length of the zoom lens as well as variations of various aberrations due to magnification-varying increase.